1. Field of the Invention
This invention relates to a reflection type optical device that is highly durable particularly in terms of moisture resistance and water resistance. The present invention is particularly suited for reflection type optical device using a transparent glass member formed by molding press.
2. Related Background Art
Reflection type optical devices prepared by coating the front or rear surface of a base member with high reflection film in order to obtain a desired image by utilizing the reflection of that surface are known. Metal film such as aluminum film or silver film showing a high reflectance is used for such high reflection film. Particularly, silver film is popularly being used for mirrors and reflection type optical devices because it shows a very high reflectance in the wavelength range of visible light between 350 nm and 700 nm.
Techniques such as vacuum evaporation, sputtering and ion plating are popular for forming metal films such as aluminum films and silver films. A wet film forming process typically involving a silver mirror reaction may be used for forming silver film. A metal film may be used by itself as high reflection film or in combination with an anti-oxidation film for preventing oxidation of the metal film and/or a reflection boosting film for improving the reflection characteristics of the metal film to produce a multilayer film structure.
Meanwhile, as a result of the current trend of down-sizing and achieving a higher performance of imaging devices, efforts for down-sizing and achieving a higher performance of reflection type optical devices to be used for such imaging devices are also demanded. As an attempt for meeting the demand, Japanese Patent Application Laid-Open No. 9-90229 proposes a method of forming an internal reflection type optical device by forming an a reflection film on a transparent base member having an aspherical optical surface. The number of lenses of an imaging device can be reduced by using the proposed method.
On the other hand, the transparent base member is required to be made to show a complex profile typically including a number of minute and irregular surfaces. Then, transparent resin or glass adapted to molding press is used as material for forming the base member of the reflection type optical device in order to accommodate the complex profile. Transparent glass is often used particularly when the light path of light passing through the inside of the internal reflection type optical device is long. Thus, glass materials adapted to molding press and having a low softening point, as well as improved optical properties, have been developed to accommodate the complex profile of the base member.
There have been developed glass materials having a low softening point and containing one or more than one of oxides including SiO2, B2O3, Al2O3, P2O5, Y2O3, La2O3, Gd2O3, TiO2, Nb2O5, ZnO, MgO, CaO, SrO, BaO, Li2O, Na2O, K2O, Sb2O3, MgF2, SnO2, ZrO2, As2O3 and GeO2 and substitution products thereof using fluorine, nitrogen or some other element.
The transparent base member is formed by heating and softening a preform prepared by cutting and polishing a gob or a glass block and subsequently press molding the preform in a metal mold having high precision surfaces. This method is characterized in that it can realize low cost and mass production because it does not require any grinding and/or polishing steps after the molding.
For the purpose of achieving the major objective of low cost mass production of reflection type optical devices, it is necessary to meet the following requirements. Firstly, the objective of low cost mass production cannot be achieved if the metal mold cannot be used repeatedly for molding press. Therefore, the temperature of molding press needs to be held as low as possible in order to avoid oxidation of the surfaces of the metal mold. While the upper limit of the temperature of molding press is between 650 and 700xc2x0 C. and accordingly the upper limit of glass transition temperature is between 550 and 600xc2x0 C. at present, it is desired to lower these upper limit values from the viewpoint of suppressing oxidation of the surfaces of the metal mold and prolonging its service life.
However, transparent base members obtained by the prior art are not satisfactory in terms of environment resistance and durability. For one thing, the chemical resistance of glass materials showing a low glass transition temperature to be used for molding is generally not satisfactory. Particularly, base members made of glass of the phosphate type, the lanthanum type or the barium type that are to be used for molding become clouded on the surface to reduce the light transmittance thereof as the glass reacts with moisture when they are stored for a long time in an environment showing high temperature and a high relative humidity.
Meanwhile, techniques of forming a coating film on the surface of a transparent base member have been proposed to address the above problem and improve the performance of the base member. For instance, Japanese Patent Application Laid-Open No. 10-139474 describes an arrangement for improving both the anti-reflection performance and the durability of a transparent base member by forming a dielectric layer having a multilayer structure (MgF2/ZrO2/MgF2/ZrO2/MgF2) on the surface of the transparent base member.
However, as a matter of course, the above arrangement of improving the environment resistance and the durability of a reflection type optical device by using a dielectric layer involves a step of forming a humidity-resistive dielectric film on the entire surface of the glass base member in addition to a step of forming a metal film on a plurality of reflection planes of the device so that the process of manufacturing such an optical device inevitably comprises a large number of steps. Additionally, the number of surfaces of an optical device on which film has to be formed has been increased remarkably as a result of meeting the requirement of realizing irregular or multiplanar surfaces to by turn increase the number of manufacturing steps. A large number of manufacturing steps entails high film forming cost.
In view of the above identified circumstances, it is therefore an object of the present invention to provide a reflection type optical device that performs well in terms of environment resistance and durability and can be manufactured at low cost by dissolving the problems of the prior art.
Another object of the present invention is to provide a method of manufacturing a reflection type optical device that performs well in terms of environment resistance and durability.
According to the invention, the above objects are achieved by providing a reflection type optical device comprising:
a transparent base member having an incident light receiving surface, at least a reflection surface for internally reflecting the incident light and a light emitting surface for emitting the reflected light;
a first dielectric layer formed on the incident light receiving surface and the light emitting surface of the transparent base member; and
a metal layer formed on all the remaining surfaces of the transparent base member other than the incident light receiving surface and the light emitting surface.
In another aspect of the invention, there is also provided a method of manufacturing a reflection type optical device comprising steps of:
preparing a transparent base member having an incident light receiving surface, at least a reflection surface for internally reflecting the incident light and a light emitting surface for emitting the reflected light;
forming a first dielectric layer on the incident light receiving surface and the light emitting surface of the transparent base member; and
forming a metal layer on all the remaining surfaces of the transparent base member other than the incident light receiving surface and the light emitting surface.